Measurements of densities within the nematic phase and of the main refractive indices at sodium-D line, both in the nematic and in the smectic phases, were done on some 4,4'-disubstituted biphenylcyclohexanes. The results are discussed in terms of the nature of the different substituents. By using the often applied Vuks model and scone bond or fragment polarisabilities the nematic order parameter, S, could be estimated.